1. Field of the Invention
The present invention generally relates to a battery protection circuit, a battery pack and apparatus using the battery pack, and more particularly, to a battery protection circuit and a battery pack for protecting a battery in electronic devices.
Recently, a demand for portable-type electronic devices is increased. Almost all portable-type electronic devices are driven by batteries. When using dry batteries, since the dry batteries are articles for consumption and are to be changed, the dry batteries don't typically have a battery protection circuit.
However, when using secondary batteries of rechargeable batteries such as NiCd, NiH batteries, since the batteries are repeatedly used by recharging, in general, such batteries have a double or a triple protection circuit. When one such battery is charged or discharged, the battery's temperature is increased, and in a worst case, battery fluid may leak out of the battery. Therefore, a set of these batteries often has a thermal protector.
Further, since over-discharging causes degradation of battery performance, it is recommended that a discharge-stop voltage be set and an over-discharging protection mechanism be provided to the battery. In a case that a cassette-type battery pack is detachable from the apparatus, a positive terminal and a negative terminal may be short-circuited to each other, which may cause rising of the battery's temperature and the fluid leak from the battery pack.
The present invention may be useful for the protection circuits in such apparatus. The present invention is not limited to the rechargeable batteries, but is applied to the dry batteries since the protection circuit may have the same function for the dry batteries.
2. Description of the Prior Art
To use batteries such an rechargeable batteries for a long time and with safety, battery protection mechanisms such as an overheating protection mechanism, an over-discharging protection mechanism, and a short-circuit protection mechanism are required.
In the following, descriptions will be given of these mechanisms in the above order.
FIG. 1 shows an example of an apparatus including a conventional overheating protection circuit. The overheating protection circuit comprises a thermistor 41 which is accompanied with a battery 40, a thermal fuse 42, a charge controller 45, and a charging circuit 44. A power source circuit 43 converts a voltage from the battery 40 to a given source voltage Vdd to be supplied to other electrical circuits. The charge controller 45 is supplied with a voltage Vt which is produced by dividing the source voltage Vdd with a resistance R and the thermistor 41.
When a surface temperature of the battery 40 extremely rises with charging, the charge controller 45 detects the overheating and controls the charging circuit 44 to stop charging the battery 40. Further, when a trouble is caused in the charge controller 45, the charging circuit 44 may go on charging the battery 40 without stopping. Therefore, the thermal fuse 42 is disposed in series along a charging path of the battery 40 to prevent the battery 40 from exploding.
FIG. 2 shows an example of a conventional battery pack. The conventional battery pack 50 comprises, for example, a battery 51, a thermistor 52 sensing a surface temperature of the battery 50, and a fuse 53. The battery pack 50 further has a positive terminal 54 and a negative terminal 56 for charging and discharging, and a thermistor terminal 55 for detecting a temperature of the battery 51 when charging the battery 51.
The battery pack 50 is usually set in such an apparatus as shown in FIG. 1 to be used. In this case, the thermistor 52 may operate in the same way as that of the thermistor 41 shown in FIG. 1. In general, a thermal protector such as a thermal fuse (not shown in FIG. 2) is prepared within the battery pack. When the temperature exceeds 70.degree. C., the thermal protector controls the battery pack to be at an open state. Further, in a case of charging the battery, the surface temperature of the battery being charged is always supervised by the charge controller, and when the temperature exceeds 50.degree. C., the charge controller stops the charging circuit from charging.
Next, a description of the over-discharging protection mechanism will be discussed. In the conventional over-discharging protection mechanism, when the battery is discharged, if a voltage detection circuit detects that the battery voltage drops less than a given value, a switching circuit may shut down the discharge operation.
Next, a description of the short-circuit protection mechanism will be discussed. In the conventional short-circuit protection mechanism, the fuse 53 is coupled in series along a discharging path as shown in FIG. 2. When the short-circuit is caused, the fuse 53 is snapped to prevent overheating and an explosion. And by forming the positive and negative terminals in specific shapes (for example, a surrounding part of the terminal metal is increased in height), it may be difficult for the terminals to contact with external elements without a specific connector.
However, the above-mentioned conventional protection mechanisms have the following disadvantages.
In the conventional overheating protection mechanism, since the thermal protector having a predetermined limiting temperature is the protection mechanism for safety of the battery, an arbitrary limitation temperature may not be set to the thermal protector according to a request from the apparatus. In this method, protection is carried out only when the charge controller and peripheral circuits operate normally to control charge. When unexpected trouble is caused in the controller and the charging circuit, the protection mechanism may not operate correctly. Also, once the thermal fuse is snapped, the battery may no longer be used although the battery itself is not broken. Thus, such a mechanism may not give a sufficient protection for the battery.
In the conventional over-discharging protection mechanism, based only on the battery voltage, the battery discharging is stopped. In General, an operational voltage at which the power source circuit is operable is not always identical to the discharge-stop voltage. For example, if the operational voltage is higher than the discharge-stop voltage, discharging goes on until the battery voltage becomes the discharge-stop voltage even if the battery voltage is in a voltage region that the power source circuit is not operable. In this voltage region, discharging of the battery is unnecessary.
In the conventional short-circuit protection mechanism with the specific terminal structure, there is still a probability of the terminal being contacted. When the terminal contacts other elements, it may spark and overheat. Though the overheating of the battery may be prevented by a fuse, there is still another problem that it takes significant effort to change the fuse to replace the snapped fuse.